1. Field of the Invention
The present invention relates to a cryogenic refrigeration system, and more particularly to a cryogenic refrigeration system with refrigerating machines applicable to a nuclear fusion apparatus and corresponding apparatus.
2. Description of the Related Art
Conventional large cryopumps are used in each of a plurality of neutral beam injection apparatuses disposed around a nuclear fusion apparatus, as disclosed in, for example, Japanese Utility Model Unexamined Publication No. 62-167875. In this case, a plurality of cryopump units are combined, and each of the cryopump units is refrigerated with liquid helium and liquid nitrogen. The cryopanels of each of the cryopump units are refrigerated to about 3.7 K. with liquid helium, and the molecules of exhaust gas are condensed and adsorbed on the cryopanels to realize high-speed exhaust of gas. A heat shield plate, refrigerated to a low temperature of about 80 K. with liquid nitrogen, is disposed around the cryopanels of each of the cryopump units so that the cryopanels are not heated directly with the radiant heat emitted from a room temperature portion or a high temperature portion disposed outside the pumps.
Liquid helium used for refrigerating the cryopanels of each of the cryopump units is produced in a large helium liquefying refrigerating machine and transferred to each of the cryopump units through thermal insulating piping such as a transfer tube. The liquid helium is evaporated in the cryopanels to generate low-temperature helium gas which is recovered by the large helium liquefying refrigerating machine through another thermal insulating piping.
Liquid nitrogen is produced in a single large nitrogen liquefying machine, or it is transferred to each of the cryopump units from a single liquid nitrogen storage tank through thermal insulating piping such as a transfer tube or the like, and is evaporated in the heat shield plate of each cryopump unit to generate low-temperature nitrogen gas. The low-temperature nitrogen gas is discharged to the atmosphere.
In the above conventional large cryopump, since liquid helium, evaporated by a small amount of heat, is transferred to each cryopump unit from the large helium liquefying refrigerating machine through the long thermal insulating piping, a large amount of liquid helium is evaporated in the thermal insulating piping. The low-temperature helium gas generated by evaporation exhibits a lower heat transfer rate than that of liquid helium, and, thus, cannot be efficiently utilized for refrigerating the cryopanels. In addition, the helium gas increases the flow pressure loss in the thermal insulating piping, thereby necessitating an increase in the diameter of the thermal insulating piping. Since the amount of the heat which enters the thermal insulating piping is generally about 1 W/m, when the length of the thermal insulating piping reaches 200 m, an amount of liquid helium corresponding to about 200 W is evaporated.
In contrast, the amount of the heat which enters a cryopump is generally several watts to several tens of watts, and most of liquid helium is consumed only for simply refrigerating the thermal insulating piping. The conventional large cryopump thus requires a great amount of liquid helium for refrigerating the thermal insulating piping, and a large helium liquefying apparatus which consumes a large amount of power is thus indispensable.
Further, if gas leaks from a portion of the thermal insulating piping in which a vacuum atmosphere is present, the thermal insulating effect is significantly decreased, and a large amount of liquid helium in the thermal insulating piping is thus evaporated, thereby causing the danger of stopping the transfer function. In addition, the thermal insulating piping cannot be refrigerated from room temperature, thereby causing trouble in that the function of the cryopump is stopped.
Further, since liquid nitrogen is transferred from the large nitrogen liquefying machine to each of the cryopump units through the long thermal insulating piping, a large amount of liquid nitrogen is evaporated in the thermal insulating piping. The low-temperature nitrogen gas generated by evaporation of liquid nitrogen in the heat shield plate is discharged to the atmosphere, and cannot be utilized for refrigerating another component. This necessitates the constant supply of a large amount of liquid nitrogen.
Namely, in the above prior art, since the liquid helium and liquid nitrogen are produced by a concentration-type large helium liquefying machine and large nitrogen liquefying machine, respectively, are transferred through the thermal insulating piping disposed in atmosphere, and are used for refrigerating the cryopump group, the prior art has the following problems:
(1) a large amount of electric power is required for refrigerating the cryopanels; PA1 (2) an expensive large helium liquefying machine and large nitrogen liquefying machine are required; PA1 (3) long and large-aperture thermal insulating piping is required; PA1 (4) a long time (several days) is required for refrigerating cryopanels because long thermal insulating piping must be previously refrigerated; PA1 (5) a long time (several days) is also required for heating cryopumps; and PA1 (6) the refrigerating system has a low reliability.
However, in a neutral beam injection apparatus used in a recent large nuclear fusion apparatus, it is necessary to decrease the power required for refrigerating the cryopumps, decrease the time required for refrigerating, and improve the reliability.